Two completely different classes of cartridges. Think the difference between .223/5.56 and .308/7.62 NATO. The 6.5 Grendel is based on the Russian military 7.62x39 case; the 6.5 Creedmoor is based on the .308 Winchester case (roughly).

The 6.5G is designed for use in the AR15 class of weapons; the 6.5C is too big for the AR15, and only fits in AR10 class weapons.

As such, the Creedmoor has more powder to shoot the same bullets faster than the Grendel.

There are a lot more specifics and good information in this AR15.com thread: